It is conventional in the construction of railroad freight cars to provide, under the floor of the car and along the centerline, a "sill" of inverted U-shaped cross section. At each end of the sill there is a cushioning device having a body or cylinder, to which is connected a car coupler and including a piston rod which extends inwardly along the sill axis, the end of the piston rod having an enlarged end collar which is held axially captive, against movement in either direction, in a box-like "backstop" of durable construction which is seated in the sill and welded or otherwise anchored to the walls of the sill.
A piston within the body of the cushioning device normally occupies a centered position, with hydraulic fluid on both sides, the piston having an orifice which develops back pressure when a shock is applied to the coupler on the cushioning device in either "buff" or "draft" direction thereby to secure cushioning action analogous to that provided in an automotive shock absorber. Since the construction described above is used almost universally in the railroad industry, efforts have been made in the design and installation to insure long life in the face of hard field usage. Thus the cushioning devices on the market are carefully manufactured using high quality steel and durable seals and piston rods of heavy cross section. Theoretically the devices should last almost indefinitely and should be able to withstand even the highest shock forces which may be encountered in practice. In actuality, however, the devices have a relatively short life, with breakage of the piston rod a frequent cause for replacement. The reason for breakage of a conservatively designed piston rod has been something of a mystery. Questions have also been raised as to the reason for premature degradation and failure of the fluid seals. Leakage at the seals is particularly troublesome in the case of so-called "gas returned" cushioning devices where a high pressure charge of compressed gas is relied upon, in lieu of a spring, for stroking the piston to centered position.